Stacking is still the best, walang masyadong pressure sa case and the discs. Kung piling, the case at the bottom of the pile might rub/squeeze on the disc. One of the problems of HD DVD discs are they are rather very fragile, and sensitive. A minor swath with a finger is enough to leave a mark. Unlike BD discs, which are impregnable to scratches, unless you really have the intention of really destroying it. Put the BD collection on the bottom kung ganon.
Thanks very much sir, ya I think mine is piled up to maybe 30 to 40 discs per column so the bottom end is heavy.